Laser imaging systems are extensively used to generate images in applications such as xerographic printing, mask and maskless lithographic patterning, laser texturing of surfaces, and laser cutting machines. Laser printers often use a raster optical scanner (ROS) that sweeps a laser perpendicular to a process direction by utilizing a polygon or galvo scanner, whereas for cutting applications laser imaging systems use flatbed x-y vector scanning.
One of the limitations of the laser ROS approach is that there are design tradeoffs between image resolution and the lateral extent of the scan line. These tradeoffs arising from optical performance limitations at the extremes of the scan line such as image field curvature. In practice, it is extremely difficult to achieve 1200 dpi resolution across a 20″ imaging swath with single galvanometers or polygon scanners. Furthermore, a single laser head motorized x-y flatbed architecture, ideal for large area coverage, is too slow for most high speed printing processes.
For this reason, monolithic light emitting diode (LED) arrays of up to 20″ in width have an imaging advantage for large width xerography. Unfortunately, present LED arrays are only capable of offering 10 milliWatt power levels per pixel and are therefore only useful for some non-thermal imaging applications such as xerography. In addition, LED bars have differential aging and performance spread. If a single LED fails it requires the entire LED bar be replaced. Many other imaging or marking applications require much higher power. For example, laser texturing, or cutting applications can require power levels in the 10 W-100 W range. Thus LED bars can not be used for these high power applications. Also, it is difficult to extend LEDs to higher speeds or resolutions above 1200 dpi without using two or more rows of staggered heads.
Higher power semiconductor laser arrays in the range of 100 mW-100 Watts do exist. Most often they exist in a 1D array format such as on a laser diode bar often about 1 cm in total width. Another type of high power directed light source are 2D surface emitting VCSEL arrays. However, neither of these high power laser technologies allow for the laser pitch between nearest neighbors to be compatible with 600 dpi or higher imaging resolution. In addition, neither of these technologies allow for the individual high speed control of each laser. Thus high power applications such as high power overhead projection imaging systems, often use a high power source such as a laser in combination with a spatial light modulator such as a DLP™ chip from Texas Instruments or liquid crystal arrays.
Prior art has shown that if imaging systems are arrayed side by side, they can be used to form projected images that overlap wherein the overlap can form a larger image using software to stitch together the image patterns into a seamless pattern. This has been shown in many maskless lithography systems such as those for PC board manufacturing as well as for display systems. In the past such arrayed imaging systems for high resolution applications have been arranged in such a way that they must use either two rows of imaging subsystems or use a double pass scanning configuration in order to stitch together a continuous high resolution image. This is because of physical hardware constraints on the dimensions of the optical subsystems. The double imaging row configuration can still be seamlessly stitched together using a conveyor to move the substrate in single direction but such a system requires a large amount of overhead hardware real estate and precision alignment between each imaging row.
For the maskless lithography application, the time between exposure and development of photoresist to be imaged is not critical and therefore the imaging of the photoresist along a single line does not need be exposed at once. However, sometimes the time between exposure and development is critical. For example, xerographic laser printing is based on imaging a photoreceptor by erasing charge which naturally decays over time. Thus the time between exposure and development is not time invariant. In such situations, it is desirable for the exposure system to expose a single line, or a few tightly spaced adjacent lines of high resolution of a surface at once.
In addition to xerographic printing applications, there are other marking systems where the time between exposure and development are critical. One example is the laser based variable data lithographic marking approach originally disclosed by Carley in U.S. Pat. No. 3,800,699 entitled, “FOUNTAIN SOLUTION IMAGE APPARATUS FOR ELECTRONIC LITHOGRAPHY”. In standard offset lithographic printing, a static imaging plate is created that has hydrophobic imaging and hydrophilic non-imaging regions. A thin layer of water based dampening solution selectively wets the plate and forms an oleophobic layer which selectively rejects oil-based inks. In variable data lithographic marking disclosed in U.S. Pat. No. 3,800,699, a laser can be used to pattern ablate the fountain solution to form variable imaging regions on the fly. For such a system, a thin layer of dampening solution also decays in thickness over time, due to natural partial pressure evaporation into the surrounding air. Thus it is also advantageous to form a single continuous high power laser imaging line pattern formed in a single imaging pass step so that the liquid dampening film thickness is the same thickness everywhere at the image forming laser ablation step. However, for most arrayed high power high resolution imaging systems, the hardware and packaging surrounding a spatial light modulator usually prevent a seamless continuous line pattern to be imaged. Furthermore, for many areas of laser imaging such as texturing, lithography, computer to plate making, large area die cutting, or thermal based printing or other novel printing applications, what is needed is laser based imaging approach with high total optical power well above the level of 1 Watt that is scalable across large process widths in excess of 20″ as well as having achievable resolution greater than 1200 dpi and allows high resolution high speed imaging in a single pass.
Traditional offset printing relies on high quality two dimensional image plates that are attached to printing cylinders that provide good single color pixel registration and consistent single color image dimensions, and that all run in lock step to provide good color-to-color image registration. A similar mechanism is required to produce a digital offset printing system that exhibits consistent and accurate single color pixel placement in order to provide good color-to-color image registration. One conventional digital offset printing system uses a high power laser based imager whose linear beam is reflected with sub-pixel spatial control onto sequentially arranged single-color imaging cylinders (drums) to form single-color two-dimensional images, which are then superimposed to produce the desired full color image. However, laser based digital offset imaging systems have a problem in that the imaging cylinders typically experience uncontrollable motion errors when forming each of the multiple single-color two dimensional images, resulting in the light energy from the laser beam reflected with sub-pixel spatial control landing on the imaging cylinder with spatial displacement errors. These motion errors generate small random displacements of the single color image “pixels” that, when superimposed to form the full color image, cause unacceptable color-to-color image registration errors that produce blurred images.
What is needed is a digital offset printing system that provides similar function to that of traditional offset printing systems to enable digital offset printing with acceptable color image registration.